<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <style>
      .box{width:50px;height:50px;background:aqua;margin:0 auto;}
    </style>
  </head>
  <body>
    <div id="box" class="box"></div>
    <script>
      var box = document.getElementById('box');
      var i = 0;                    
      box.onclick = function() {     
        ++i;
        if (i % 2) {                 
          this.style.width = '200px';
          this.style.height = '200px';
          this.innerHTML = '大';
        } else {                    
          this.style.width = '50px';
          this.style.height = '50px';
          this.innerHTML = '小';
        }
      };
    </script>
     <p><a href="第七章.html">返回上一级</a></p>
  </body>
</html>